1. Field of the Invention
The present invention is related to sensing devices using laser spectroscopy, and more particularly, is related to headspace sensing and analysis of sealed optically-transparent containers that are moved along a conveyor.
2. Background and Material Information
Tunable diode laser absorption spectroscopy (TDLAS) is a high sensitivity measurement method well-suited to monitoring gases in small volume containers for manufacturing process and quality control applications.
For automated package inspection applications where gas concentrations and/or pressures need to be measured in individual product containers at high speed, a spectroscopic system must be designed to move individual samples through a measurement region, and the sensing system is designed to automatically self-calibrate and perform self diagnostics.
Most TDLAS systems are configured for analyzing gas samples using stationary sample cells and reference cells. A typical system is described by COOPER et al. (U.S. Pat. No. 5,572,031) where a single laser beam is split into two laser beams using a beam splitter. In such a system, one optical path passes through the sample cell (containing e.g., a gas to be measured, tested and/or checked) and the second passing through a reference cell. The light directed along the reference and sample paths are then collected by separate detectors.
The art is replete with TDLAS systems similar to COOPER that employ stationary dual detectors and stationary sample and reference cells. In such systems, the laser beam is split and passed separately through a sample cell and reference cell. This arrangement has many shortcomings and undesirable effects. First, the use of beam splitting optics in TDLAS systems leads to optical interference fringes that reduce measurement sensitivity, especially when testing for weakly-absorbing samples, such as oxygen. Second, the use of two detectors, one detector to calibrate and a second detector to sample leads to uncertainties about the validity of the system calibration in the sample path. Third, the use of fixed sample and reference cells necessitates that the sample gas be introduced to the sample cell for measurement. This last point is particularly undesirable if the sample is contained in a finished product ready for shipment as opposed to a process stream that can be sampled.
Many new and existing commercial products, e.g., pharmaceutical, food, beverage and consumer electronics, have ingredients that are sensitive to atmospheric gases. Prolonged exposure to elevated levels of these gases (e.g., oxygen and moisture) degrades product quality and potency. When processing gas-sensitive formulations, ambient levels of reactive gases are reduced with inert gas purges or packages are sealed under vacuum. Thus, a fast, sensitive, and nondestructive method and apparatus for continuous monitoring of gas concentrations and pressures in sealed product containers as they move along a conveyor is desired.